Phase fraction measurement using light source adjusted in discrete steps

ABSTRACT

Disclosed herein is an apparatus including a structure containing a multiphase fluid and having a transparent window. A collimated light source emits light through the transparent window structure at a wavelength at which a component of a desired phase of the multiphase fluid is absorptive. A photodetector is positioned such that the emitted light passes through the multiphase fluid in the structure and out through the transparent window structure to impinge upon the photodetector. The photodetector has an actual dynamic range for light detection. Processing circuitry adjusts a power of the collimated light source in a series of steps dependent upon a relationship between an output level of the photodetector and a threshold to cause measurement of the emitted light over an effective dynamic range greater than the actual dynamic range. Properties of the multiphase fluid are determined as a function of the measured emitted light.

BACKGROUND

This invention is related to the field of phase fraction measurement ofa multiphase fluid, and, more particularly, to a multiphase flowmeterfor making such phase fraction measurements.

DESCRIPTION OF THE RELATED ART

In hydrocarbon production, fluid produced from wells and flowing throughvarious points of a production system is multiphasic. Thus, the fluidmay have any of a gas phase, an oil phase, a water phase, and a solidphase. For a variety of reasons, it can be desirable to know thefraction of the fluid represented by each phase. For example, thepresence of solids, such as sand, in fluid can result in the erosion ofequipment of a production system and even damage.

Multiphase flowmeters may be used to determine these phase fractions. Amultiphase flowmeter may be installed in tubing and may employ aradioactive source and scintillation detector to enable measurement ofthe phase fractions. Such multiphase flowmeters have proven to beaccurate and desirable.

Due to challenging regulatory environments, however, the fact that amultiphase flowmeter utilizes a radioactive source may be undesirable.In addition, current multiphase flowmeters may be prohibitivelyexpensive for use in wells with relatively low rates of production.

Therefore, a commercial desire exists for further development in thearea of multiphase flowmeters.

SUMMARY

Certain aspects of some embodiments disclosed herein are set forthbelow. It should be understood that these aspects are presented merelyto provide the reader with a brief summary of certain forms theinvention might take and that these aspects are not intended to limitthe scope of the invention. Indeed, the invention may encompass avariety of aspects that may not be set forth below.

Disclosed herein is an apparatus including a pipe through which amultiphase fluid flows with a transparent window structure formed in thepipe. A collimated light source is configured to emit light through thetransparent window structure and into the pipe with the emitted lighthaving a wavelength at which a component of a desired phase of themultiphase fluid is absorptive. A photodetector is positioned such thatthe emitted light passes through the multiphase fluid in the pipe andout through the transparent window structure to impinge upon thephotodetector. The photodetector has an actual dynamic range forcollimated light detection. Processing circuitry coupled to thecollimated light source and photodetector is configured to continuouslyadjust a power of the collimated light source dependent upon an outputlevel of the photodetector so as to cause measurement of the emittedlight by the photodetector over an effective dynamic range greater thanthe actual dynamic range. The processing circuitry also determines atleast one property of the multiphase fluid as a function of the power ofthe collimated light source.

A method aspect is directed to a method of determining at least oneproperty of a multiphase fluid. The method includes emitting collimatedlight into the multiphase fluid, with the emitted light having awavelength at which a component of a desired phase of the multiphasefluid is absorptive. Light passing through the multiphase fluid thatimpinges upon a photodetector having an actual dynamic range forcollimated light detection is detected. A power of the collimated lightsource is continuously adjusted dependent upon an output level of thephotodetector so as to cause measurement of the emitted light by thephotodetector over an effective dynamic range greater than the actualdynamic range. At least one property of the multiphase fluid isdetermined as a function of the power of the collimated light source.

Another aspect is directed an apparatus including a structure containinga multiphase fluid, with a transparent window structure formed in thestructure. A collimated light source is configured to emit light throughthe transparent window structure and into the structure, with theemitted light having a wavelength at which a component of a desiredphase of the multiphase fluid is absorptive. A photodetector ispositioned such that the emitted light passes through the multiphasefluid in the structure and out through the transparent window structureto impinge upon the photodetector. The photodetector has an actualdynamic range for collimated light detection. Processing circuitry iscoupled to the collimated light source and photodetector. The processingcircuitry is configured to adjust a power of the collimated light sourcein a series of steps dependent upon a relationship between an outputlevel of the photodetector and at least one threshold so as to causemeasurement of the emitted light by the photodetector over an effectivedynamic range greater than the actual dynamic range. The processingcircuitry is also configured to determine at least one property of themultiphase fluid as a function of the measured emitted light.

Another method aspect is directed to a method of determining at leastone property of a multiphase fluid. The method includes emittingcollimated light into the multiphase fluid, with the emitted lighthaving a wavelength at which a component of a desired phase of themultiphase fluid is absorptive. Light passing through the multiphasefluid and impinging upon a photodetector having an actual dynamic rangefor collimated light detection is detected. A power of the collimatedlight source is adjusted in a series of steps dependent upon arelationship between an output level of the photodetector and at leastone threshold so as to cause measurement of the emitted light by thephotodetector over an effective dynamic range greater than the actualdynamic range. At least one property of the multiphase fluid isdetermined as a function of the measured emitted light.

Still another aspect is directed to an apparatus that includes a pipethrough which a multiphase fluid flows, with a transparent windowstructure formed in the pipe. A collimated light source is configured toemit light through the transparent window structure and into the pipe,where the emitted light has a wavelength at which a component of adesired phase of the multiphase fluid is absorptive. A photodetector ispositioned such that the emitted light passes through the multiphasefluid in the pipe and out through the transparent window structure toimpinge upon the photodetector. The photodetector has an actual dynamicrange for collimated light detection. Processing circuitry is coupled tothe collimated light source and photodetector and configured to adjust apower of the collimated light source so as to cause measurement of theemitted light by the photodetector over an effective dynamic rangegreater than the actual dynamic range. The processing circuitry is alsoconfigured to determine at least one property of the multiphase fluid asa function of the measured emitted light and/or the power of thecollimated light source

Various refinements of the features noted above may exist in relation tovarious aspects of the present embodiments. Further features may also beincorporated in these various aspects as well. These refinements andadditional features may exist individually or in any combination. Forinstance, various features discussed below in relation to theillustrated embodiments may be incorporated into any of theabove-described aspects of the present disclosure alone or in anycombination. Again, the brief summary presented above is intended justto familiarize the reader with certain aspects and contexts of someembodiments without limitation to the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a phase fraction determinationsystem capable of discriminating among three phases, in accordance withthis disclosure.

FIG. 2 is a schematic block diagram of a phase fraction determinationsystem capable of discriminating among four phases, in accordance withthis disclosure.

FIG. 3 is a schematic block diagram of a phase fraction determinationsystem capable of sand detection, in accordance with this disclosure.

FIG. 4 is a schematic block diagram of a phase fraction determinationsystem similar to that of FIG. 2 but with an additional collimated lasersource to assist with discrimination among four phases.

FIG. 5 is a graph showing transmission absorption optical density forvarious hydrocarbon oils and water.

FIG. 6 is another graph showing transmission absorption optical densityfor various hydrocarbon oils and water.

FIG. 7 is a further graph showing transmission absorption opticaldensity for various hydrocarbon oils and water.

The figures are not to scale. Wherever possible, the same referencenumbers will be used throughout the drawing(s) and accompanying writtendescription to refer to the same or like parts.

DETAILED DESCRIPTION

It is to be understood that the present disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below for purposes of explanation and to simplify thepresent disclosure. These are, of course, merely examples and are notintended to be limiting.

When introducing elements of various embodiments, the articles “a,”“an,” “the,” and “said” are intended to mean that there are one or moreof the elements. The terms “comprising,” “including,” and “having” areintended to be inclusive and mean that there may be additional elementsother than the listed elements. Moreover, any use of “top,” “bottom,”“above,” “below,” other directional terms, and variations of these termsis made for convenience, but does not mandate any particular orientationof the components.

Referring initially to FIG. 1, a phase fraction determination system 100is now described. The system includes a pipe or tube 102 through which amultiphase fluid flows. The multiphase fluid may have any of three orfour phases, which include a gas phase, oil phase, a water phase, andsolid phase. The gas phase and oil phase are hydrocarbon bearing. “Gas”is used here to denote any form of hydrocarbon bearing gas, and “oil” isused generically here to denote any form of hydrocarbon oil.

As shown, the cross section of the pipe 102 is circular, although insome applications, a pipe 102 with a rectangular or square cross sectionmay be used. The pipe 102 may have a relatively small radius, forexample on the order of 5 mm-30 mm; where the pipe 102 has a rectangularcross section, the dimensions may be on the order of 10 mm-30 mm×2 mm-6mm, for example, or even larger. Where the pipe 102 has a rectangularcross section, the cross sectional area can be increased for the samepath length as in a pipe 102 with a circular cross section. Othersuitable radiuses and dimensions may be used in appropriate applicationsas well as other structures in addition to pipe 102.

A window 104 is formed in the pipe 102. The window 104 is illustrativelycylindrical in shape, although other shapes may be used. The window 104may be formed from synthetic sapphire. Where the pipe 102 is rectangularin cross section, the window 104 may be rectangular in cross section, ormay be two separate windows located on opposite sides of the pipe 102.The window 104 is optically transparent to the wavelengths of collimatedlight emitted by the collimated light sources, such as, for example,lasers 106 a-106 c, as will now be explained.

The collimated light sources or lasers 106 a-106 c are illustrativelylaser light sources employing laser diodes and emitting laser light inthe near-infrared wavelength spectrum, although other types of laserlight sources may be employed in some applications. Each laser 106 a-106c emits light in a different narrow wavelength spectrum. For example,laser 106 a emits light having a wavelength of λ1, corresponding to awavelength where oil is substantially more absorptive than water.

Dichroic mirrors 108 a-108 c respectively combine the collimated lightfrom the lasers 106 a-106 c on an optical path through the window 104and into the multiphase fluid. It should be noted that where the crosssection of the pipe 102 is rectangular, the mirrors 108 a-108 c areconfigured to direct the collimated light from the lasers 106 a-106 cacross the shorter path length available in the rectangular crosssection, and not the longer path.

The lasers 106 a-106 c need not be perfectly aligned along the opticalpath, and would function effectively if they were not aligned along theoptical path but converged on and focused on the same spot of thephotodetector 116. Thus, in some applications, mirrors that are notdichroic may be used.

In some applications, rather than dichroic mirrors 108 a-108 c, otherdevices may be used to reflect and combine the collimated light from thelasers 106 a-106 c. Such other device may be polarized beam splitters orunpolarized beam splitters, for example.

In the optical path from the lasers 106 a-106 c, a beam splitter 107 maybe used to split the power of the lasers. A small power-fraction beam111 measured by a photodetector 117 (with a thermally-stable opticalattenuator if useful) may monitor the variations in laser output overtime and compensate for power changes due to temperature variations anddevice aging. The majority of the power from the lasers 106 a-106 cpasses through the beam splitter 107, and is delivered to the window 104through a series of cylindrical lenses 110 a-110 c that serve to shapethe emitted light into a nearly two dimensional ribbon 112 shape, forexample having dimensions of 15 mm×0.5 mm, that passes throughsubstantially all, or a substantial majority of, or an entire crosssection of, the cross section of the pipe 102. A lens 114, on the farside of the pipe 102, focuses the two dimensional light ribbon 112 as itexits the pipe 102 for detection by a photodetector 116. From thespectrum of light impinging on the photodetector 116, or from the outputpower of the lasers 106 a-106 c monitored by the photodetector 117,phase fractions of the multiphase fluid can be determined by theprocessing circuitry 130, which receives output signals from thephotodetectors. The photodetectors 116 and 117, as well as thecollimated light sources 106 a-106 c, may be thermally stabilized byactive cooling or heating to achieve stable measurements.

In order to enable the photodetectors 116 and 117 to separately andindependently measure the intensities of the outputs from the lasers 106a-106 c, the lasers 106 a-106 c are temporally multiplexed such that butone of the lasers 106 a-106 c is emitting light at once. Stated anotherway, the temporal multiplexing results in laser 106 a emitting lightwhile lasers 106 b-106 c are not, laser 106 b emitting light whilelasers 106 a, 106 c are not, and laser 106 c emitting light while lasers106 a-106 b are not. A limitation of this temporal multiplexing is thatthe flow rate of the multiphase fluid should be substantially less thanthe total time elapsed for each laser 106 a-106 c to activate once inturn. For example, where each laser 106 a-106 c is activated for 30 μs,the total interrogation time is thus 90 μs. Where the ribbon ofcollimated light 112 has a thickness of 0.5 mm, the instantaneousvelocity of the multiphase fluid should remain less than 1.4 m/s.

Alternatively, instead of temporal multiplexing, the lasers 106 a-106 cmay be operated simultaneously but at different pulsing frequencies.These frequencies are chosen so as to be reasonably far away frommultiphase fluid fluctuation frequencies in order to enable the use ofphase-sensitive detection (PSD) of the light impinging on thephotodetectors 116 and 117 by the processing circuitry 130 to providefor discrimination between components of the light impinging on thephotodetectors 116 and 117 that were emitted by the first laser 106 a,components of the light impinging upon the photodetectors 116 and 117that were emitted by the second laser 106 b, and components of the lightimpinging upon the photodetectors 116 and 117 that were emitted by thethird laser 106 c. By using PSD, the processing circuitry 130 alsofunctions to attenuate erroneous readings from the photodetectors 116and 117 caused by stray light events.

The use of PSD may realize multi-wavelength measurements simultaneouslyand for multiphase flows at the same location in the pipe. Higherquality signals may be obtained by, as explained, rejecting stray lightsautomatically by PSD, and phase fractions may be measured moreaccurately due to the simultaneous data detection at multiplewavelengths.

As will be explained in detail below, the processing circuitry 130, inaddition to determining the phase fractions of the multiphase fluid,acts to control the lasers 106 a-106 c as a function of the lightdetected by the photodetector 116.

Details of operation will now be given with reference to FIG. 5. Auseful wavelength for λ₁ is a hydrocarbon peak absorption band at whichhydrocarbons, such as oil and gas, are a few times to a few thousandtimes more absorptive of light than water. A useful wavelength for λ₂ isat a lower end of a water peak absorption band, at which water issubstantially more absorptive of light than hydrocarbons, such as oiland gas. In addition, a useful wavelength for λ₃ is a band at whichneither water nor oil is substantially absorptive.

As an example, as shown in FIG. 5, λ₁ may be at the hydrocarbon peak,which is between 1693 nm and 1757 nm, for example in some cases at 1727nm, and at which hydrocarbons such as oil and gas are a few times to afew thousand times more absorptive of light than water. λ₂ may be at thelower end of the water peak or water absorption band, around 1565 nm,where water is substantially more absorptive of light than hydrocarbonssuch as oil and gas. λ₃ may be at 1350 nm, which is at a relatively lowpoint on the absorption spectrum of interest, such that any attenuationor absorption can be understood to be due to bubbles, droplets, orsolids, and at which components of the oil and water phases are notsubstantially absorptive. Thus, in some embodiments, the thirdcollimated light source or laser 106 c may have a third wavelength atwhich components of the first and second phases of the multiphase fluidare not absorptive.

The wavelengths λ₁, Δ₂, and λ₃ are such that the optical densities ofthe phases of the multiphase fluid do not exceed undesirable amountswhich would render the emitted collimated light undetectable underattenuation. Shown in FIG. 5 is a graph of optical density (OD) verseswavelength including the above discussed wavelengths λ₁, λ₂, and λ₃. Ascan be seen from FIG. 5, the optical densities at the wavelengths λ₁,λ₂, and λ₃ do not exceed 0.5 per millimeter path length. Using a pipe102 having a diameter of 19 mm, the optical density could reach 9.5,which correlates to an attenuation of 109.5 times. With a diode laseremitting light at a 350 mW output power, after attenuation,approximately 0.1 mW of output power could be detected, which isdetectable by conventional InGaAs, InAsSb, or germanium photodetectorswith an acceptable signal to noise ratio. A similar example may be foundin FIG. 7. Here, the hydrocarbon peak is around 1630 nm.

For certain heavy oils, however, the optical density at the wavelengthsλ₁, λ₂, and λ₃ is greater than those shown in FIG. 5. Therefore, in someinstances, different and more appropriate values for λ₁, λ₂, and λ₃ maybe used. Here, λ₁ may be selected as 1725 nm, λ₂ may be selected as 2211nm, and λ₃ may be selected as 1823 nm, as shown in FIG. 6. However, theoptical density at these wavelengths λ₁, λ₂, and λ₃ is on the order of0.8 per millimeter, meaning that to obtain the same detection of 0.1 mWof output power, the diameter of the pipe 102 has to be reduced from 19mm to 12 mm. An alternative to reduction of diameter of the pipe 102 isto use an avalanche photodiode or photomultiplier tube as thephotodetector 116. It should be noted that where the cross section ofthe pipe 102 is rectangular, the collimated light at the wavelengths λ₁,λ₂, and λ₃ may be made to measure the optical density across the shorterpath length (such as 5 mm) in the rectangular cross section, and notacross the longer path length (such as 22 mm). The phase fractiondetermination system 100 may adjust the wavelengths of the lasers basedon knowledge from oil samples.

As should be appreciated, the photodetector 116 (or the photodetector117) has a dynamic range of light intensity that is capable of detectingup to 3 to 4 times the optical density range of state-of-the-artphotodetectors. However, to determine phase fractions over a full gasvolume fraction range, light intensities outside the actual dynamicrange of the photodetector 116 are to be detected. Therefore, as will bedescribed below, techniques may be employed in order to operate thephotodetector 116 or 117 with an effective dynamic range that is greaterthan the actual dynamic range.

The actual dynamic range of the photodetector 116 or 117 is thedifference between the lowest intensity of light that the photodetector116 or 117 can detect and the highest intensity of light that thephotodetector 116 or 117 can discriminate. Light having an intensitygreater than the highest intensity of light that the photodetector 116or 117 can discriminate, or light having an intensity less than thelowest intensity of light that the photodetector 116 or 117 can detect,is said to be outside of the actual dynamic range of the photodetector116 or 117. Stated another way, the actual dynamic range of thephotodetector 116 or 117 is the difference between the smallest andlargest usable signal producible by the photodetector 116 or 117.

The effective dynamic range of the photodetector 116 or 117 can be madeto be greater than the actual dynamic range using the techniquesdescribed herein. The above effective dynamic range extension isachieved by continuously adjusting the output power of the lasers 106a-106 c in real time (which is monitored by the photodetector 117 withthe use of known optical attenuation means if desired), as a function ofthe output level of the photodetector 116 such that the output level ofthe photodetector 116 remains constant. It should be appreciated thatdesigns where there are an equal number of photodetectors 116 to thenumber of lasers 106 a-106 c are within the scope of this disclosure.

When operating as per this first technique, since the intensity/powerlevel at the photodetector 116 is constant while the output power levelof the lasers 106 a-106 c fluctuates (and is monitored by thephotodetector 117), the processing circuitry 130 monitors the currentconsumption or power level of the lasers 106 a-106 c in order todetermine the phase fractions of the multiphase fluid. Due to thecontinuous real time adjustment of the output power of the lasers 106a-106 c, the photodetector 116 is prevented from saturating. Thus,independent of the attenuation provided by the various phases of themultiphase fluid, the lasers 106 a-106 c may at times have output powersoutside of the actual dynamic range of the photodetector 116. Since thelasers 106 a-106 c are monitored to determine the phase fractions of themultiphase fluid and since the photodetector 116 is prevented fromsaturating, the output power of the lasers 106 a-106 c may at timesexceed the actual dynamic range of the photodetector 116, yetmeasurements may still accurately be made. Due to this technique, thenonlinearity of the photodetector 116 may be irrelevant and have noeffect on results.

A second technique for operating the photodetector 116 at an effectivedynamic range greater than the actual dynamic range is where theprocessing circuitry 130 adjusts the output power of the lasers 106a-106 c in a series of steps dependent upon a relationship between anoutput level of the photodetector and at least one threshold. In greaterdetail, the processing circuitry 130 switches the output power of thelasers 106 a-106 c from a higher level (or step) to a lower level (orstep) in a discrete step when the output level of the photodetector 116exceeds an upper threshold, and switches the output power of the lasers106 a-106 c from a lower level (or step) to a higher level (or step) ina discrete step when the output level of the photodetector 116 fallsbelow a lower threshold. Thus, the output level of the lasers 106 a-106c remains constant between the higher threshold and lower threshold.

A specific example of this second technique is now described. Forexample, for a 19 mm diameter pipe 102, it is desirable for thephotodetector 116 to be capable of detecting light intensities from 0.1mW to 350 mW, which corresponds to nine orders of magnitude. Yet, theactual dynamic range of the photodetector 116 is merely five orders ofmagnitude. This desired effective dynamic range of nine orders ofmagnitude may thus be divided into three intensity stages, each of whichcovers three orders of magnitude. As the processing circuitry 130detects the photodetector 116 nearing saturation (thus, the output ofthe photodetector 116 rises beyond an upper threshold), it discretelyreduces the output power of the lasers 106 a-106 c to a lower level.Similarly, when the processing circuitry 130 detects the output of thephotodetector 116 falling below a lower threshold, it discretelyincreases the output power of the lasers 106 a-106 c to a higher level.

Here, the processing circuitry 130 analyzes the outputs of thephotodetectors 116 and 117 in order to determine the phase fractions ofthe multiphase fluid. In addition, as with the first technique, sincethe photodetector 116 is prevented from saturating, the output power ofthe lasers 106 a-106 c may at times exceed the actual dynamic range ofthe photodetector 116, yet measurements may still accurately be made.

Processing of either the output V_(λ) of the photodetector 116, which isa DC voltage proportional to the light intensity I_(λ) detected by thephotodetector 116, or output power I_(o,λ) of the lasers 106 a-106 cmeasured by the photodetector 117, by the processing circuitry 130 isnow described. For a given wavelength λ, the light intensity signalacquired from the photodetector 116 I_(λ) or the lasers 106 a-106 cI_(o,λ) is proportional to the intensity of the transmitted light(I_(λ)). Accounting for scattering due to bubbles/droplets, this can bemathematically represented using the Beer-Lambert law, and can bewritten as:I _(λ) =I _(0,λ) [e ^(−Σ) ^(i) ^(α) ^(i) ^(χ) ^(i,λ) ^(d) K(α_(i),ζ,λ,d)],where iϵ{oil, water, gas}, χ_(i) represents the linear attenuationcoefficient of the phases obtained from a fluid-sample referencemeasurement, and K(α_(i), ζ, λ, d) represents a general scattering termwhich is dependent on the phase fractions α_(i), bubble/droplet size andgeometrical factor ζ, wavelength λ, and pipe diameter d. ExcludingRayleigh scattering (i.e. scattering by particles much smaller than thewavelength), the scattering term K would generally be independent ofwavelength.

Using the signal from λ₃, the scattering term (K) can be factored out,and the set of equations reduces to the following:

${\begin{bmatrix}\chi_{w,{\lambda 1}} & \; & \chi_{w,{\lambda 3}} & \; & \chi_{o,{\lambda 1}} & \; & \chi_{o,{\lambda 3}} & \; & \chi_{g,{\lambda 1}} & \; & \chi_{g,{\lambda 3}} \\\chi_{w,{\lambda 2}} & \; & \chi_{w,{\lambda 3}} & \; & \chi_{o,{\lambda 2}} & \; & \chi_{o,{\lambda 3}} & \; & \chi_{g,{\lambda 2}} & \; & \chi_{g,{\lambda 3}} \\\; & 1 & \; & \; & \; & 1 & \; & \; & \; & 1 & \;\end{bmatrix} \cdot \begin{bmatrix}\alpha_{w} \\\alpha_{o} \\\alpha_{g}\end{bmatrix}} = {\quad{{\begin{bmatrix}{R\left( {I_{\lambda 1},I_{\lambda 3}} \right)} \\{R\left( {I_{\lambda 2},I_{\lambda 3}} \right)} \\1\end{bmatrix}\mspace{20mu}{where}},\mspace{20mu}{{R\left( {I_{\lambda 1},I_{\lambda 3}} \right)} = {{\frac{1}{d}{\ln\left( {\frac{I_{0,{\lambda 1}}}{I_{0,{\lambda 3}}}\frac{I_{\lambda 3}}{I_{\lambda 1}}} \right)}\mspace{20mu}{R\left( {I_{\lambda 2},I_{\lambda 3}} \right)}} = {\frac{1}{d}{\ln\left( {\frac{I_{0,{\lambda 2}}}{I_{0,{\lambda 3}}}\frac{I_{\lambda 3}}{I_{\lambda 2}}} \right)}}}}}}$

The matrix on the left is called the relative attenuation matrix wherethe scattering effect has been subtracted. The values of the matrixelements can be obtained from full-bore or calibration-cell measurementson each phase (i.e. in-situ reference). Once the matrix is known, thephase fraction could be calculated upon inversion of this matrix. Forrobustness, this matrix is to be invertible and to have a determinantmuch larger than zero. Those of skill in the art will understand thatthe equations above are an example for the case where determination ofthe phase fraction for three phases is performed, but that theseequations can be generalized to account for a fourth phase, such assand. In fact, these equations can be generalized to account for anynumber of phases, such as a fifth phase that is hydrogen sulfide.

The system 100 described above is useful for determining the phasefractions of gas, oil, and water. However, in some instances, themultiphase fluid may include a solid phase (such as sand particles), andit may be desirable to know the phase fraction of the solid phase aswell. To that end, the system 100 may be modified to measure the fourth(solid) phase as will now be described with reference to FIG. 2.

Added to the system 100 in this embodiment is a lens 118 perpendicularto the lenses 110 a-100 c and 114, that serves to focus light 113reflected or scattered from solids 122 or sand in the multiphase fluidfor detection by an additional photodetector 120. Through the functionand calculations described above, together with data from the additionalphotodetector 120, the processing circuitry 130 may determine a phasefraction for the solid phase.

During operation, the output of the photodetector 116 (and thus theintensity of collimated light impinging on the photodetector 116) ismonitored and measured over time by the processing circuitry 130. Sincethe intensity of collimated light impinging on the photodetector 116fluctuates over time due to the phase composition of the multiphasefluid, and in particular due to the presence of solids 122 or sandgrains, and bubbles or droplets 124, this measured intensity over timeyields a pattern of intensity values, where some intensity values aregreater than others.

By performing analysis on this pattern of intensity values and matchingthe pattern of intensity values to known patterns, the processingcircuitry 130 can determine the presence of sand grains. For example,using a pattern correlation, fitting, or matching technique, such aschi-square or residual sum, the processing circuitry 130 may, in thetime domain, compare the pattern of intensity values to a knownintensity value pattern or set of patterns that indicate presence ofsolids 122 or sand grains. Where the measured pattern of intensityvalues matches the known intensity value pattern or set of patterns, theprocessing circuitry 130 may determine from the amplitude and durationof the matched pattern that solids 122 or sand grains are present. Inaddition, matching the measured pattern of intensity values to a knownintensity value pattern can yield information about the solids 122 orsand grains themselves. For example, there may be different knownintensity value patterns for different types or sizes of solids 122 orsand grains, or for different fractional percentages of the solids 122or sand grains in the multiphase fluid. Thus, by matching the measuredpattern of intensity values to a known intensity value pattern, inaddition to determining that solids 122 or sand grains are present, atype of solids 122 or sand grains, size of the solids 122 or sandgrains, number of the solids 122 or sand grains, or fractionalpercentage of the solids 122 or sand grains in the multiphase fluid maybe determined. The known intensity value patterns may be a prioriknowledge gathered experimentally, through modeling, or from a database.

Rather than analyzing the measured pattern of intensity values in thetime domain, the processing circuitry 130 may instead analyze thepattern of intensity values in the frequency domain. Therefore, theprocessing circuitry 130 may perform a Fourier transform on the measuredpattern of intensity values to yield a pattern or spectrum of measuredfrequencies. A filtering may then optionally be applied by theprocessing circuitry 130 to the spectrum of measured frequencies to asto reject certain frequencies, and the result (or the original spectrum,in the case where filtering is not performed) compared to a knownfrequency pattern or set of patterns. Where the spectrum of measuredfrequencies matches the known frequency pattern or set of patterns, theprocessing circuitry 130 determines that solids 122 or sand grains arepresent.

Matching the spectrum of measured frequencies to a known frequencypattern can yield information about the solids 122 or sand grainsthemselves. For example, there may be different known frequency patternsfor different types or sizes of solids 122 or sand grains, or fordifferent fractional percentages of the solids 122 or sand grains in themultiphase fluid. Thus, by matching the spectrum of measured frequenciesto a known frequency pattern, in addition to determining that the solids122 or sand grains are present, a type of the solids 122 or sand grains,size of the solids 122 or sand grains, number of solids 122 or sandgrains, or fractional percentage of the solids 122 or sand grains in themultiphase fluid may be determined. The known frequency patterns may bea priori knowledge gathered experimentally, through modeling, or from adatabase.

Stated another way, the analysis performed by the processing circuitry130 may be used to identify presence of a frequency band in the spectrumof measured frequencies that indicates the presence of solids 122 suchas sand. From this, the phase fraction of solids 122 or sand may bedetermined. In making this determination, data from an empirical model(itself based on experiment and simulation) may be combined with thedetermined sizes and numbers of solids 122 present.

In some applications, the processing circuitry 130 may perform analysison the pattern of measured intensity values in both the time domain andfrequency domain, and correlate the results to one another so as toimprove accuracy of the pattern matching.

As explained above, this measured intensity over time yields a patternof intensity values, where some intensity values are greater thanothers. Thus, stated another way, the pattern of intensity valuesincludes drops in intensity. As explained, where a solid 122, such as agrain of sand, passes through the collimated light, the photodetector116 registers a drop in intensity for a short period of time. The dropin intensity may be a substantial decrease in intensity, and may begreater than 5%, 15%, or 20% for example. The drop in intensity may alsoor instead be a drop in intensity during a window of interest, as willbe explained below. Through the analysis described above, the durationof this intensity drop can be correlated to the size of the solid 122,while the number of intensity drops can be correlated to the number ofsolids 122 or sand grains present.

In correlating the duration of the drop in intensity to the size of thesolids 122 or sand grains, the diameters of the solids 122 or sandgrains is estimated as a function of the duration of the intensity drop.From the diameters of the solids 122 or sand grains, the processingcircuitry 130 can, where desired, calculate a total volume of solids 122or sand grains.

In the case where the processing circuitry 130 continuously adjusts thepower of the lasers 106 a-106 c dependent upon the output level of thephotodetectors 116, the processing circuitry 130 accordingly increasesthe power of the lasers 106 a-106 c, such that the power of the lasers106 a-106 c increases suddenly in correspondence to the intensity dropregistered by the photodetector 116. Thus, in this application, theprocessing circuitry 130 measures and analyzes the power output of thelasers 106 a-106 c over time, and the operations and analysis describedabove can be performed on the pattern of measured power outputs of thelasers 106 a-106 c.

Thus, the duration of this power increase or decrease of the lasers 106a-106 c can be correlated to the size of the solids 122 or sand grains,while the number of power increases or decreases of the lasers 106 a-106c can be correlated to the number of solids 122 or sand grains present.In correlating the duration of the power increase of the lasers 106a-106 c to the size of the solids 122 or sand grains, the diameters ofthe solids 122 or sand grains is estimated as a function of the durationof the power increase of the lasers 106 a-106 c. From the diameters ofthe solids 122 or sand grains, the processing circuitry 130 can, wheredesired, calculate a total volume of solids 122 or sand grains.

The monitoring and measurement of the output of the photodetector 116,as described above, may be performed by the processing circuitry 130.Optionally, the photodetector 116 may use an estimate of theinstantaneous velocity of the solids 122 or sand grains (determinedseparately from cross-correlation velocimetry and/or laser Dopplermeasurement) to determine a window of interest in which the intensitydrops, or measured patterns (in either time domain or frequency domain)are to occur if they are to indicate presence of the solids 122 or sandgrains.

The specific use of the light 113 reflected or scattered from solids 122or sand grains in the multiphase fluid, as detected by photodetector120, will now be discussed. In addition to solids 122, bubbles ordroplets 124 may also result in an intensity drop, or change in themeasured pattern of intensity values, as they pass through thecollimated light. To distinguish between solids 122 or sand grains, andbubbles or droplets 124, the photodetector 120 is used to detect light113 scattered from solids 122 or sand grains, and/or bubbles or droplets124, as the scattering behavior of solids 122 such as sand grains isdifferent than that of bubbles or droplets 124. The processing circuitry130 performs spectral analysis to distinguish whether the scatteredlight 113 was scattered from the solids 122 or sand grains, or thebubbles and droplets 124, and the results of this spectral analysis maybe taken into account when performing the time domain and frequencydomain pattern matching described above so as to provide for a moreaccurate determination of the solid phase fraction of the multiphasefluid by the processing circuitry 130, and for a more accuratedetermination of the gas phase fraction of the multiphase fluid by theprocessing circuitry 130, since light 113 scattering from bubbles 124indicates the presence of bubbles of gas.

Yet another way to determine the phase fraction of the solids 122 orsand grains is through the use of a machine learning setup, such as aneutral network employing a reinforcement learning algorithm. Theneutral network is set up by being fed known training data, and fromthat, learns how to determine the phase fractions of the solids 122 orsand grains by monitoring the output of the photodetectors 116 and 120.

A fourth laser 106 d and its corresponding dichroic mirror 108 d mayalso be used in sand detection, as shown in the embodiment of FIG. 4.Here, the fourth laser 106 d also emits laser light along the opticalpath through the window 104 and into the multiphase fluid. Thewavelength λ₄ of the fourth laser 106 d is chosen to be close to, or at,an absorption peak of the phase of interest, such as, for example, sandspecies of interest, hydrogen sulfide, or any other desired phase, butat which wavelength λ₄ the optical density of the multiphase fluid doesnot exceed an amount that would render the emitted laser lightundetectable under attenuation. The data on detected attenuation due tosand absorption, or absorption of any other phase of interest, can beused by the processing circuit 130 to enhance the outputs of the sand,or solid 122, phase fraction determination as described above, using thetechniques described.

Any number of additional lasers may be used, with each laser emitting ata wavelength chosen to be close to, or at, an absorption peak of a phaseof interest. Thus, it is to be understood that the system disclosedherein is capable of determining the phase fraction of any number ofphases of a multiphase fluid.

Since sand can be particularly damaging to production systems, in someapplications it may be desirable to make a solid detector fordetermining a phase fraction of solids 122 within a multiphase flow, butwithout determining the phase fractions of gas, oil, and water. To thatend, one embodiment shown in FIG. 3 may include the third laser 106 d,but not the other lasers. Since temporal multiplexing need not be usedin this embodiment, the third laser 106 d may directly emit light alongthe optical path toward the window 104, and a mirror need not bepresent. This embodiment functions to detect solids 122 and to determinethe phase fractions of solids 122 in the multiphase fluid from thescattered light 113 as described above with reference to FIG. 3.

In the specification and appended claims: the terms “connect,”“connection,” “connected,” “in connection with,” and “connecting” areused to mean “in direct connection with” or “in connection with via oneor more elements;” and the term “set” is used to mean “one element” or“more than one element.” Further, the terms “couple,” “coupling,”“coupled,” “coupled together,” and “coupled with” are used to mean“directly coupled together” or “coupled together via one or moreelements.” As used herein, the terms “up” and “down,” “upper” and“lower,” “upwardly” and downwardly,” “upstream” and “downstream;”“above” and “below;” and other like terms indicating relative positionsabove or below a given point or element are used in this description tomore clearly describe some embodiments of the disclosure.

Although the preceding description has been described herein withreference to particular means, materials and embodiments, it is notintended to be limited to the particulars disclosed herein; rather, itextends to all functionally equivalent structures, methods, and uses,such as are within the scope of the appended claims. Many modificationsand other embodiments will come to the mind of one skilled in the arthaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is understoodthat various modifications and embodiments are intended to be includedwithin the scope of the appended claims.

The invention claimed is:
 1. An apparatus comprising: a structurecontaining a multiphase fluid; a transparent window structure formed inthe structure; a collimated light source configured to emit lightthrough the transparent window structure and into the structure, theemitted light having a wavelength at which a component of a desiredphase of the multiphase fluid is absorptive; a photodetector positionedsuch that the emitted light passes through the multiphase fluid in thestructure and out through the transparent window structure to impingeupon the photodetector; wherein the photodetector has an actual dynamicrange for collimated light detection; and processing circuitry coupledto the collimated light source and the photodetector, the processingcircuitry configured to: adjust a power of the collimated light sourcein a series of steps dependent upon a relationship between an outputlevel of the photodetector and at least one threshold so as to causemeasurement of the emitted light by the photodetector over an effectivedynamic range greater than the actual dynamic range; and determine atleast one property of the multiphase fluid as a function of the measuredemitted light.
 2. The apparatus of claim 1, wherein the processingcircuitry adjusts the power of the collimated light source in the seriesof steps dependent upon the relationship between the output level of thephotodetector and the at least one threshold by: switching the power ofthe collimated light source from a higher step to a lower step when theoutput level of the photodetector exceeds an upper threshold; andswitching the power of the collimated light source from the lower stepto the higher step when the output level of the photodetector fallsbelow a lower threshold.
 3. The apparatus of claim 1, wherein theemitted light passes through substantially an entire cross sectionalarea of the multiphase fluid in the structure.
 4. The apparatus of claim1, further comprising at least one lens associated with the collimatedlight source for shaping the emitted light into a ribbon shape.
 5. Theapparatus of claim 1, wherein the collimated light source comprises afirst collimated light source having a first wavelength at which acomponent of a first phase of the multiphase fluid is absorptive; andfurther comprising a second collimated light source having a secondwavelength at which a component of a second phase of the multiphasefluid is absorptive, and a third collimated light source having a thirdwavelength at which components of the first and second phases of themultiphase fluid are not absorptive.
 6. The apparatus of claim 5,wherein the first phase of the multiphase fluid comprises oil; whereinthe second phase of the multiphase fluid comprises water; wherein athird phase of the multiphase fluid comprises gas; and wherein the atleast one property of the multiphase fluid comprises phase fractions ofgas, oil, and water.
 7. The apparatus of claim 5, wherein the firstwavelength is substantially at a hydrocarbon peak absorption band;wherein the second wavelength is at a water absorption band where wateris more absorptive of light than oil; and wherein the third wavelengthis at a band where neither water nor oil is substantially absorptive. 8.The apparatus of claim 5, wherein the processing circuitry is configuredto differentiate between sand, and bubbles or droplets.
 9. The apparatusof claim 5, wherein the processing circuitry is configured to pulse thefirst, second, and third collimated light sources in a pattern such thatbut one of the first, second, and third collimated light sources isemitting light at a time.
 10. The apparatus of claim 5, wherein a fourthphase of the multiphase fluid comprises a solid phase; furthercomprising a fourth collimated light source having a fourth wavelengthat which components of the solid phases of the multiphase fluid areabsorptive.
 11. The apparatus of claim 5, wherein the processingcircuitry is configured to pulse the first, second, and third collimatedlight sources simultaneously and at different pulsing frequencies fromone another, and is configured to perform phase-sensitive detectionsimultaneously for the first, second, and third collimated lightsources.
 12. The apparatus of claim 11, wherein the processing circuitryis configured to discriminate between components of the emitted lightimpinging upon the photodetector that were emitted by the firstcollimated light source, components of the emitted light impinging uponthe photodetector that were emitted by the second collimated lightsource, and components of the emitted light impinging upon thephotodetector that were emitted by the third collimated light source,using the phase-sensitive detection.
 13. The apparatus of claim 5,wherein: the first collimated light source comprises a first laser lightsource configured to emit laser light along a path that does not includethe transparent window structure, and a first mirror configured toreflect the laser light from the first laser light source along a paththat includes the transparent window structure; the second collimatedlight source comprises a second laser light source configured to emitlaser light along a path that does not include the transparent windowstructure, and a second mirror configured to reflect the laser lightfrom the second laser light source along a path that includes thetransparent window structure; and the third collimated light sourcecomprises a third laser light source configured to emit laser lightalong a path that does not include the transparent window structure, anda third mirror configured to reflect the laser light from the thirdlaser light source along a path that includes the transparent windowstructure; and further comprising: at least one lens to focus thereflected laser light from the first, second, and third laser lightsources.
 14. The apparatus of claim 1, further comprising an additionalphotodetector positioned such that emitted light that scatters off themultiphase fluid and out through the transparent window structureimpinges upon the additional photodetector; wherein the processingcircuit is configured to measure the scattered emitted light; whereinthe processing circuitry also determines the at least one property ofthe multiphase fluid as a function of the measured scattered emittedlight; and wherein the at least one property comprises a phase fractionof solids in the multiphase fluid.
 15. A method of determining at leastone property of a multiphase fluid comprising: emitting collimated lightfrom a collimated light source into the multiphase fluid, the emittedcollimated light having a wavelength at which a component of a desiredphase of the multiphase fluid is absorptive; measuring the emittedcollimated light by detecting light passing through the multiphase fluidimpinging upon a photodetector having an actual dynamic range forcollimated light detection; adjusting a power of the collimated lightsource in a series of steps dependent upon a relationship between anoutput level of the photodetector and at least one threshold so as tocause measurement of the emitted collimated light by the photodetectorover an effective dynamic range greater than the actual dynamic range;and determining at least one property of the multiphase fluid as afunction of the measured emitted collimated light.
 16. The method ofclaim 15, wherein adjusting the power of the collimated light source inthe series of steps dependent upon the relationship between the outputlevel of the photodetector and the at least one threshold comprises:switching the power of the collimated light source from a higher step toa lower step where the output level of the photodetector exceeds anupper threshold; and switching the power of the collimated light sourcefrom the lower step to the higher step where the output level of thephotodetector falls below a lower threshold.
 17. The method of claim 15,wherein measurement of the emitted collimated light by the photodetectorover time yields a pattern of intensity values; and wherein determiningthe at least one property of the multiphase fluid as a function of themeasured emitted collimated light comprises determining presence of sandgrains as a function of the pattern of intensity values.
 18. The methodof claim 17, wherein the presence of the sand grains is determined as afunction of the pattern of intensity values by performing a Fouriertransform on the pattern of intensity values, and determining thepresence of the sand grains by matching a frequency pattern output bythe Fourier transform to a known sand grain frequency pattern.
 19. Themethod of claim 17, wherein the presence of the sand grains isdetermined as a function of the pattern of intensity values bycorrelating the pattern of intensity values to a known sand grainintensity value pattern.
 20. The method of claim 17, further comprisingdetermining at least one of sizes of the sand grains and a number of thesand grains as a function of the pattern of intensity values.